SAN FRANCISCO — It was just another California rush hour. On eastbound Interstate 580 near Pleasanton, a rocking chair brought traffic to a near-standstill, while on southbound I-680 near Walnut Creek, a trampoline blocked two left lanes, wreaking havoc with the morning commute. Bagged loaves of sourdough bread blocked U.S. 101, near Petaluma. The highway patrol had to be dispatched.

Last month a plastic sink stretched across two lanes of the San Mateo Bridge. The sink was something of a departure from the mattresses, aluminum ladders, sofas, buckets — with the occasional spilled Napa County grapes thrown in — that snarled traffic and contributed to the state’s daily accumulation of road debris.

While by no means unique to California, pickup trucks and other vehicles piled high with improperly secured loads are a fact of life here, contributing to an estimated

140,000 cubic yards of road debris a year.That is enough trash to fill

8,750 garbage trucks, which would extend for 45 miles, said Tamie McGowen, a spokeswoman for Caltrans. It is increasingly hazardous, experts said.

In California, 155 people lost their lives in the last two years after accidents involving objects on highways, and states are beginning to address the issue. Next week a murder trial is set to begin in the death of a Los Angeles county deputy sheriff killed when he swerved to avoid a stolen stove that fell from a Long Beach man’s truck.

In California and across the nation, where some freeway shoulders have come to resemble weekend yard sales, the nature of road debris has changed, and litter anthropologists are now studying the phenomenon. Where “deliberate” litter used to reign — those blithely tossed fast food wrappers and the like — “unintentional” or “negligent” litter from poorly secured loads is making its presence felt.

By dint of its climate, size, population, lengthy growing season, increasingly long commutes and, perhaps, its casual lifestyle, California is a road-debris leader. It is also home to the country’s largest number of registered vehicles — 32 million, twice that of Texas — and roughly 4 million pickup trucks, the most of any state, according to the Alliance of Automobile Manufacturers in Washington.

No state spends more on litter removal, in excess of $55 million, said Christine Flowers-Ewing, the executive director of Keep California Beautiful, a nonprofit environmental education organization.

Motorists here can be fined if anything other than feathers from live birds or water should escape (in Nebraska, the exception is corn stalks; in Kentucky, coal).

Along with mudslides, brush fires and earthquakes, chance encounters with a set of box springs, a chintz cushion or a crate of lettuces are the daily stuff of radio traffic updates, recounted in excruciating detail.

When a tractor-trailer full of freeze-dried Oriental soup and vegetables spilled onto the 710 freeway near Los Angeles, for example, “there was one from Column A, and one from columns B and C,” said Peter Demetriou, a veteran traffic reporter for KFWB radio in Los Angeles. “The only problem was, it was Lane 1, Lane 2 and Lane 3.”

A 2004 report on vehicle-related road debris by the AAA Foundation for Traffic Safety underscored the hazards: In North America, more than

25,000 accidents a year are caused by litter that is dumped by motorists or falls out of vehicles.

Theories on the psychology of unintentional litter vary. Timothy W. Jones, a materials anthropologist at the University of Arizona, speculates that the do-it-yourself phenomenon of self-hauling, particularly home improvement goods, is a contributing factor, as are high landfill fees that tempt some people to discard items on the road.

It may also result from more aggressive tailgating and higher speeds that prompt cargo to become airborne, says John D. Schert, executive director of the Hinkley Center for Solid and Hazardous Waste Management at the University of Florida (www.litterinfo.org).

The arguably American traits of optimism and denial — “the nothing-bad-is-going-to-happen-to-me mentality” — also affect the trend, said P. Wesley Schultz, a professor of psychology at California State University, San Marcos, who studies littering. “People tend to feel more secure in their vehicles than they really are” at 65 miles per hour, he said.

Litter escalates when the temperature is 50 to 75 degrees, and declines on rainy days, said Daniel Syrek, a litter consultant in Sacramento. One little-understood aspect of road debris is the percentage generated by commercial versus private vehicles, the subject of a survey to be conducted in Georgia, where the construction boom has contributed to the problem.

“We’re a more itinerant society than we used to be,” said Schert at the University of Florida. “People don’t have a sense of ownership about where they live.”

For those who brave California’s freeways, errant rakes and brooms may be a continuing challenge of life behind the wheel. Phil Linhares, chief curator of art at the Oakland Museum of California, who drives frequently between San Francisco and Los Angeles, recalled his own near miss with a chrome dinette set recently.

Linhares makes it a point to drive defensively on the California litter gridiron. “People are real casual here,” he said. “If it’s not raining, someone says, ‘Let’s get some string and tie the box spring to the roof rack,'” he said. “It’s carelessness and utter stupidity.”

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